Spectrometers are used for analysing the wavelength distribution of the intensity of optical radiation. In scanning grating monochromators and prism monochromators the radiation to be measured is supplied into the device through an entrance slit and the radiation is dispersed by a grating and a prism by supplying one wavelength band at a time to a single element detector and registering the intensity spectrum of the radiation to be measured directly as a function of the movement of the grating or mirror. The device measures only one wavelength band at a time and thus wastes most of the optical power available. Furthermore, this device requires a very accurate mechanical movement and its measurement, which are expensive. The device also wears in use and is sensitive to thermal expansion, dirt and vibration. Inaccuracy of movement of the grating and the drift of wavelength scale caused by it is a considerable error factor when the device is used for long-term quantitative or qualitative analysis of the chemical composition of samples in the NIR area (Near InfraRed).
In Fourier Transform Infra Red (FTIR) devices the radiation to be measured is led through a Michelson interferometer, the optical arms of which determine the difference in optical path length which is altered by a moving mirror during the measurement. The intensity of radiation that has passed the interferometer is measured as a function of the difference in optical path length by means of a single element detector, and the intensity distribution thus obtained as the function of the difference in optical path length is called an interferogram. The intensity spectrum of radiation is calculated from the interferogram using a Fourier transform. The FTIR device measures all wavelengths simultaneously. Thanks to multiplexing, the FTIR device provides a considerably better signal-to-noise ratio than the scanning grating monochromator if the detector noise is the dominant source of noise, which is usually the case in the IR area. Since this technique requires an interferometer which includes a moving mirror, it is extremely sensitive to interference in the environment, such as vibration and temperature changes. Furthermore, modulation resulting from sample movement causes interference as a moving sample is measured.
The scanning grating monochromator can be implemented without the problems caused by the mechanical movement by forming the grating by an acoustic wave in an AOTF component (Acousto Optical Tunable Filter). Scanning is performed by changing the grating constant, and thus its angular dispersion is changed by altering the frequency of the acoustic wave. This solution is, however, expensive, generates only a small amount of optical power and does not provide the advantage of multiplexing.
In a grating spectrograph the entrance slit is imaged onto the surface of a multi-element (e.g. 16 to 1024) row detector via the grating so that the place of the entrance slit image moves in the row detector in the longitudinal direction of a row (from one element to another) as a function of the wavelength, in which case each of the detector elements registers a separate wavelength band. No moving parts are needed in such a spectrograph and it provides the advantage of multiplexing. However, the row detector needed in the IR area in this solution is expensive, and furthermore, signal detection requires expensive and complex detection electronics. Change of the wavelength area or resolution requirement often entails a new expensive row detector, which may constitute a vicious circle. Concentration measurement devices based on absorption spectroscopy apply a method called ‘ratio measurement’, which is used for eliminating changes of the radiation source temperature, measurement geometry, scattering and detector response by calculating an estimate for the concentration of the substance to be measured from the ratio of the intensities measured at the absorption wavelength of the substance to be measured and at a reference wavelength selected from its side, in which case coefficient errors independent of the wavelength are eliminated in the division. Since the wavelength responses of different elements in the row detector vary due to the deficiencies in the manufacturing process, their temperatures change at slightly different rates, which causes errors dependent on the wavelength and time. For this reason thermal stabilization of the row detector is considerably more difficult than that of a single element detector and leads to expensive and heavy solutions in device implementations.
New kind of cheaper spectrometers have been implemented by modulating different wavelength channels selectively with a deformable micromirror device DMD provided in the place of the row detector of the grating spectrograph or with another component suitable for spatial modulation. Modulated optical signals are imaged to the detector and the intensities of the wavelength channels to be measured are detected by demodulating a signal measured by the detector. Since the grating that separates the wavelengths from one another spreads the image of the entrance slit on the modulator surface, the image to be produced therefrom is still too large, and thus the device requires a detector with a large surface area, which is expensive, often difficult to obtain and also has too high a capacitance, which results in slow function of the detector. The fact that the detector is also dependent on the temperature also causes problems in this solution and thermal stabilization is needed. Such a solution is described more closely in Batchelor, J. D., Jones, B. T.: Development of a Digital Micromirror Spectrometer for Analytical Atomic Spectrometry, Analytical Chemistry, Vol. 70, No. 23, pp. 4907 to 4914, Dec. 1, 1998, which is incorporated herein by reference.
Very cheap miniature spectrographs have been produced in large series using various mass production techniques (LIGA, etc.), but the problem related to these devices is that diffused light, which scatters from poor-quality optical surfaces and contains all wavelengths, spreads on the row detector surface and forms a level below which optical powers cannot be detected reliably. This restricts the measurable variation range of the concentration of an absorbing substance. The LIGA technique is explained in greater detail e.g. in P. Kripper, J. Mohr, C. Müller, C. van der Sel, Microspectrometer for the Infrared Range, SPIE vol. 2783, pages 277 to 282, 1996, which is incorporated herein by reference. The LIGA technique is also described in Handbook of Microlitography, Micromachining and Microfabrication, P. Rai-Choudhury, ed., vol. 2 Micromachining, and Microfabrication, pages 237 to 377, 1997.